Publication of Japanese patent application 2009-65246 (JP-A-2009-65246) discloses a conventional load control apparatus configured to control a duty ratio of electricity supply so as to regulate fluctuation in a load, even when a voltage of a power source changes. Specifically, the load control apparatus supplies electricity such that a duty ratio becomes small as the effective value of electric power becomes large, thereby to regulate increase in an effective value of electric power of the load.
For example, a relay control circuit is configured to control supply of electricity to a coil of a relay switch thereby to control activation and deactivation of the relay switch. In general, such a relay control circuit is configured to perform a duty control of an energization period of a coil, when voltage of a power source changes, thereby to supply constant electricity to the coil.
It is noted that, a wattage (effective value of electric power) of a load, which is a filament-type lamp, is not in proportion to an illumination caused by the filament-type lamp. Accordingly, when the load control apparatus of JP-A-2009-65246 is employed as a lamp control circuit for controlling a filament-type lamp as a load, the lamp control circuit may be incapable of controlling the illumination of the filament-type lamp constantly. As a result, illumination of the lamp may be flickering.
A relay control circuit is configured to perform a duty control of an energization period. It is noted that increase in a coil current supplied to a coil may delay by a damping time constant. Therefore, even when the relay control circuit sets an energization period according to change in a voltage (power supply voltage) of a power source, a coil current may become insufficient. In such a case, electricity supply may become insufficient, and magnetism caused by the coil may change. Thus, the relay control circuit may become incapable of driving the relay device or maintaining activation of the relay device.
FIG. 12 is a time chart showing an ON-OFF waveform (activation and deactivation waveform) of a coil switch and a coil current waveform when a power supply voltage changes. As shown in FIG. 12, when the power supply voltage is in a normal voltage, a coil current increases to a desired current. Thus, the coil can be supplied with predetermined electric current. When the power supply voltage increases, the duty control may reduce the energization period. In this case, the energization period may end before the coil current reaches the desired current. Consequently, the coil current may become insufficient, and desired electric current may not be supplied to the coil.
In the present case, a relay control circuit is mentioned as one example of a circuit configured to supply electric current to a coil (load element). It is noted that, similar problem occurs not only in such a relay control circuit but also in a load control circuit configured to perform a duty control of electric current supplied to a coil (load element) thereby to activate and deactivate the coil.
As described above, a lamp control circuit may cause fluctuation (flickering) in lamp illumination, or a relay control circuit may cause fluctuation in magnetism generated by a coil. Consequently, desired state cannot be maintained in various load control circuits. Thus, useless power consumption may occur. In recent years, a gasoline vehicle (automotive) is substituted to, for example, a hybrid vehicle or an electric vehicle. Such a hybrid vehicle or an electric vehicle consumes a large amount of electricity. Thus, reduction in useless electric consumption is strongly desired. In consideration of such a background, reduction in useless electric consumption in a load control circuit is also desired.